Investigation into the Dynamic Fracture Properties of Large Scale Functionally Graded Materials

Abstract:

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A crack propagation perpendicular to gradient in a large scale functionally gradient
materials, which has (1) a linear variation of Young’s modulus with a constant mass density and
Poisson’s ratio, and (2) a exponential variation of Young’s modulus with a constant mass density
and Poisson’s ratio, is modelled by finite element methods. Based on the experimental result of
large scale functionally gradient materials, the dynamic propagation process of the FGMs is
modelled and the dynamic parameters, like the energy release rate and crack tip opening angle, are
calculated through a generation phase.

Abstract: In this study the three – dimensional surface cracking of a graded coating bonded to a homogeneous substrate is considered. The main objective is to model the subcritical crack growth process in the coated medium under a cyclic mechanical or thermal loading. Because of symmetry, along the crack front conditions of mode I fracture and plane strain deformations are assumed to be satisfied. Thus, at a given location on the crack front the crack propagation rate would be a function of the mode I stress intensity factor. A three – dimensional finite element technique for
nonhomogeneous elastic solids is used to solve the problem and the displacement correlation technique is used to calculate the stress intensity factor.

Abstract: Preventing pipeline from rapid crack propagation is a critical issue to avoid casualties and disasters. In this paper, by combining the energy balance theory with FEM simulation and arrest criteria, the numerical analysis is developed to solve the problem of crack dynamic propagation in gas pipeline. This simulation, in combination with the full-scale blast tests, provides a broad prediction of the dynamic fracture process. The crack tip opening angle (CTOA) criterion is consummated through the comparison between CTOA in FEM calculation and the critical value of (CTOA)C obtained by the experiment. The result of the simulation for the crack speed and location is consistent with data by Alliance and Japanese full-scale blast tests.

Abstract: The viscosity of material is considered at propagating crack-tip. Under the assumption
that the artificial viscosity coefficient is in inverse proportion to the power law of the plastic strain
rate, an elastic-viscoplastic asymptotic analysis is carried out for moving crack-tip fields in
power-hardening materials under plane-strain condition. A continuous solution is obtained
containing no discontinuities. The variations of the numerical solution are discussed for mode I
crack according to each parameter. It is shown that stress and strain both possess exponential
singularity. The elasticity, plasticity and viscosity of material at the crack-tip only can be matched
reasonably under linear-hardening condition. The tip field contains no elastic unloading zone for
mode I crack.

Abstract: Crack growth path was investigated experimentally, numerically and theoretically using two test specimens subjected to pure mode II loading. The specimens were (a) the center cracked circular disc (CCCD) specimen subjected to diametral compression often called the Brazilian disc and (b) the diagonally loaded square plate (DLSP) specimen containing inclined center crack and subjected to pin loading. A few CCCD and DLSP specimens made of two brittle materials (i.e. marble rock and PMMA) were tested under pure mode II conditions. It was observed that the fracture initiation directions and the fracture paths for the tested specimens differed significantly and grew in two different trajectories. However, it was shown that the experimentally observed fracture paths for both specimens can be predicted theoretically very well by using the incremental crack growth method. Several finite element analyses were performed to simulate the whole fracture trajectories of the tested CCCD and DLSP specimens. At each increment, the direction of fracture initiation for the tip of growing crack was determined using the fracture parameters (i.e. stress intensity factors and T-stress) based on the modified maximum tangential stress (MMTS) criterion. The main difference in the fracture trajectory was found to be related to the magnitude and sign of the fracture parameters (which depend strongly on the specimen geometry and loading configuration) and also the type of tensile or compressive loading in the CCCD and DLSP samples.

Abstract: Based on the theoretical study on the tip stress intensity factor (SIF) of the crack normal to and dwelling on the interface, using the finite element software ANSYS, the SIFs of the double interface cracks normal to and dwelling on the interface in cladding material structure are studied by changing the crack spacing, the crack length, the cladding thickness ratio, the load and the crack location. The results show that, the crack SIFs become larger with the increase of the crack spacing, the crack length and the load, they become smaller with the increase of the coating thickness ratio, that the SIF of the crack close to the boundary becomes smaller with the increase of the distance between the crack and the boundary, and that the SIF of the crack in the middle of the interface becomes larger with the decrease of the crack distance.